Forests

A forest is a large area of land covered with trees or other woody vegetation. Forests are the dominant terrestrial ecosystem of Earth, and are distributed across the globe.

Forests at different latitudes form distinctly different ecozones: boreal forests near the poles tend to consist of evergreens, while tropical forests near the equator tend to be distinct from the temperate forests at mid-latitude. The amount of precipitation and the elevation of the forest also affects forest composition.

Types of Forest

Tropical rainforests

Sub-tropical forests

Mediterranean forests

Coniferous forests

Temperate forests

Plantation forests

Montane forests

Tropical rainforest

Hugely dense, lush forest with canopies preventing sunlight from getting to the floor of the forest.

All year high temperatures and abundant rainfall.

Located near the equator.

A vital storehouse of biodiversity, sustaining millions of different animals, birds, algae and fish species.

Sub-tropical forest

Located at the south and north of the tropical forest.

Trees here are adapted to resist the summer drought.

Mediterranean forest

Located at the south of the temperate regions around the coasts of the Mediterranean, California, Chile and Western Australia.

The growing season is short and almost all trees are evergreen, but mixed with hardwood and softwood.

Temperate forest

Located at Eastern North America, Northeastern Asia, and western and eastern Europe.

Mix of deciduous and coniferous evergreen trees.

Usually, the broad-leaved hardwood trees shed leaves annually.

There are well-defined seasons with a distinct winter and sufficient rainfall.

Coniferous forest

Located in the cold, windy regions around the poles.

They come in both hardwoods and conifers.

The hardwoods are deciduous.

The conifers are evergreen and structurally adapted to withstand the long drought-like conditions of the long winters.

Montane forest

Known as cloud forest because they receive most of their precipitation from the mist or fog that comes up from the lowlands.

Usually found in high-elevation tropical, subtropical and temperate zones.

Plants and animals in these forests are adapted to withstanding the cold, wet conditions and intense sunlight.

Earth Atmosphere layers

The atmosphere of Earth is the layer of gases surrounding the planet Earth that is retained by Earth’s gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention, and reducing temperature extremes between day and night.

The common name air is given to the atmospheric gases used in breathing and photosynthesis. By volume, dry air contains 78.09%nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere. Air content and atmospheric pressure vary at different layers, and air suitable for the survival of terrestrial plants and terrestrial animals is found only in Earth’s troposphere.

The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The Kármán line, at 100 km is often used as the border between the atmosphere and outer space. Severallayers can be distinguished in the atmosphere, based on characteristics such as temperature and composition.

Structure of the atmosphere

Principal layers

In general, air pressure and density decrease with altitude in the atmosphere. Earth’s atmosphere can be divided (called atmospheric stratification) into five main layers. From highest to lowest, the five main layers are:

Exosphere: 700 to 10,000 km (440 to 6,200 miles)

Thermosphere: 80 to 700 km (50 to 440 miles)

Mesosphere: 50 to 80 km (31 to 50 miles)

Stratosphere: 12 to 50 km (7 to 31 miles)

Troposphere: 0 to 12 km (0 to 7 miles)

Exosphere

The exosphere is the outermost layer of Earth’s atmosphere. It extends from the exobase, which is located at the top of the thermosphere at an altitude of about 700 km above sea level, to about 10,000 km. The exosphere merges with the emptiness of outer space, where there is no atmosphere.

This layer is mainly composed of extremely low densities of hydrogen, helium and several heavier molecules including nitrogen, oxygen and carbon dioxide closer to the exobase. The exosphere is located too far above Earth for any meteorological phenomena to be possible. The exosphere contains most of the satellites orbiting Earth.

Thermosphere

The thermosphere is the second-highest layer of Earth’s atmosphere. It extends from the mesopause (which separates it from the mesosphere) at an altitude of about 80 km up to the thermopause at an altitude range of 500–1000 km. The lower part of the thermosphere, from 80 to 550 kilometres above Earth’s surface, contains the ionosphere.This atmospheric layer undergoes a gradual increase in temperature with height. A person would not feel warm because of the thermosphere’s extremely low pressure.This layer is completely cloudless and free of water vapor.

Mesosphere

The mesosphere is the third highest layer of Earth’s atmosphere, occupying the region above the stratosphere and below the thermosphere. It extends from the stratopause at an altitude of about 50 km to the mesopause at 80–85 km above sea level.

Temperatures drop with increasing altitude to the mesopause that marks the top of this middle layer of the atmosphere. It is the coldest place on Earth and has an average temperature around −85 °C (−120 °F; 190 K).

It is too high above Earth to be accessible to aircraft and balloons, and too low to permit orbital spacecraft. The mesosphere is mainly accessed by sounding rockets.

Stratosphere

The stratosphere is the second-lowest layer of Earth’s atmosphere. It lies above the troposphere and is separated from it by the tropopause. This layer extends from the top of the troposphere at roughly 12 km above Earth’s surface to the stratopause at an altitude of about 50 to 55 km.

It contains the ozone layer, which is the part of Earth’s atmosphere that contains relatively high concentrations of that gas. The stratosphere defines a layer in which temperatures rise with increasing altitude. This rise in temperature is caused by the absorption of ultraviolet radiation (UV) radiation from the Sun by the ozone layer, which restricts turbulence and mixing. Although the temperature may be −60 °C (−76 °F; 210 K) at the tropopause, the top of the stratosphere is much warmer, and may be near 0 °C.

This is the highest layer that can be accessed by jet-powered aircraft.

Troposphere

The troposphere is the lowest layer of Earth’s atmosphere. It extends from Earth’s surface to an average height of about 12 km, although this altitude actually varies from about 9 km (30,000 ft) at the poles to 17 km (56,000 ft) at the equator with some variation due to weather. The troposphere is bounded above by the tropopause, a boundary marked in most places by a temperature inversion and in others by a zone which is isothermal with height.

The temperature usually declines with increasing altitude in the troposphere because the troposphere is mostly heated through energy transfer from the surface. Thus, the lowest part of the troposphere (i.e. Earth’s surface) is typically the warmest section of the troposphere. The troposphere contains roughly 80% of the mass of Earth’s atmosphere.

Nearly all atmospheric water vapor or moisture is found in the troposphere, so it is the layer where most of Earth’s weather takes place

Structure of Earth

The interior structure of the Earth is layered in spherical shells, like an onion. Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core.

The structure of mechanically can be divided into

lithosphere,

asthenosphere

mesospheric mantle

outer core

inner core

The interior of Earth is divided into 5 important layers. Chemically, Earth can be divided into

Crust

upper mantle

lower mantle

outer core

Inner core

Depth

Layer

Kilometres

Miles

0–60

0–37

Lithosphere (locally varies between 5 and 200 km)

0–35

0–22

… Crust (locally varies between 5 and 70 km)

35–60

22–37

… Uppermost part of mantle

35–2,890

22–1,790

Mantle

100–200

210-270

… Upper mesosphere (upper mantle)

660–2,890

410–1,790

… Lower mesosphere (lower mantle)

2,890–5,150

1,790–3,160

Outer core

5,150–6,360

3,160–3,954

Inner core

Core

The average density of Earth is 5,515 kg/m3. Since the average density of surface material is only around 3,000 kg/m3, we must conclude that denser materials exist within Earth’s core. Seismic measurements show that the core is divided into two parts, a “solid” inner core with a radius of ~1,220 km[3] and a liquid outer core extending beyond it to a radius of ~3,400 km.

The inner core is generally believed to be composed primarily of iron and some nickel. It is not necessarily a solid, but, because it is able to deflect seismic waves, it must behave as a solid in some fashion.

The liquid outer core surrounds the inner core and is believed to be composed of iron mixed with nickel and trace amounts of lighter elements.

Mantle

Earth’s mantle extends to a depth of 2,890 km, making it the thickest layer of Earth. The upper mantle is divided into the lithospheric mantle and the asthenosphere. The upper and lower mantle are separated by the transition zone. The lowest part of the mantle next to the core-mantle boundary is known as the D″ layer. The mantle is composed of silicate rocks that are rich in iron and magnesium relative to the overlying crust

Crust

The crust ranges from 5–70 km in depth and is the outermost layer. The thin parts are the oceanic crust, which underlie the ocean basins (5–10 km) and are composed of dense (mafic) iron magnesium silicate igneous rocks, like basalt. The thicker crust is continental crust, which is less dense and composed of (felsic) sodiumpotassium aluminium silicate rocks, like granite.